In general, a repeater is installed in a region where a signal from a main transmitter is received at a weak level, and it can solve an unstable reception and expand a coverage area of the main transmitter.
FIG. 1 is a block view explaining an example of a conventional repeating system where repeaters use different frequencies.
Referring to FIG. 1, a main transmitter 101 transmits a signal at a frequency A, and repeaters 102 to 105 relay the signal at frequencies B, C, D and E different from the frequency A. Since the repeaters 102 to 105 use the different frequencies B, C, D and E in the conventional repeating system, multiple frequency bands must be ensured for configuration of the repeating system. Using multiple frequency resources for the repeating system is very inefficient in the aspect of the frequency use.
If multiple repeaters use the same frequency as the main transmitter, an effect of frequency reuse can be obtained even over a short distance, thereby improving frequency use efficiency.
FIG. 2 is a view for explaining another example of a conventional repeating system. In FIG. 2, repeaters are on-channel repeaters using the same frequency.
That is, a main transmitter 201 transmits a signal at a frequency A, and repeaters 202 to 205 relay the signal at the frequency A.
To allow such on-channel repeating, signals of the same frequency respectively transmitted from the main transmitter 201 and the on-channel repeaters 202 to 205 must be distinguishable from each other.
If output signals of the same frequency band, i.e., output signals of the main transmitter 201 and the on-channel repeaters 202 to 205 are not identical, those output signals act as an on-channel interference signal at each repeater, and are not removed by any equalizer or another device.
If signals from the on-channel repeaters 202 to 205 have a delay longer than a predetermined reference with respect to a signal from the main transmitter 201, an equalizer fails to remove the delayed signals.
Therefore, for the on-channel repeating, output signals of the on-channel repeaters 202 to 205 must be identical to an output signal of the main transmitter 201, and the time delay between the two output signals must be small.
Limitations of the conventional on-channel repeaters will now be described with reference to FIGS. 3 to 7.
FIG. 3 is a block diagram showing one example of a conventional RF amplification on-channel repeater.
Configurations and operations of the conventional RF amplification on-channel repeater will now be described with reference to FIG. 3. An Rx antenna 301 and an RF receiver 302 receive an RF signal transmitted from a main transmitter. An RF band-pass filter 303 passes only a predetermined signal band of the received RF signal. A high-power amplifier 304 amplifies the passed RF signal, and the amplified RF signal is transmitted via a Tx antenna 305 over the same channel.
FIG. 4 is a block diagram for explaining an example of a conventional intermediate-frequency (IF) conversion on-channel repeater.
Configurations and operations of the conventional IF conversion on-channel repeater will now be described with reference to FIG. 4. An Rx antenna 401 and an RF receiver 402 receive an RF signal transmitted from a main transmitter. An IF down-converter 403 down-converts the received RF signal into an IF signal based on a reference frequency provided from a local oscillator (LO) 408.
The IF band-pass filter 404 passes only a predetermined band signal of the down-converted IF signal. An RF up-converter 405 up-converts the passed IF signal into an RF signal based on a reference frequency provided from the LO 408. A high-power amplifier 406 amplifies the up-converted RF signal, and the amplified RF signal is transmitted via a Tx antenna 407 over the same channel.
FIG. 5 is a block diagram of an example of a conventional surface acoustic wave (SAW) filter on-channel repeater.
Configurations and operations of the conventional SAW filter on-channel repeater will now be described with reference to FIG. 5. An Rx antenna 501 and an RF receiver 502 receive an RF signal transmitted from a main transmitter. An IF down-converter 503 down-converts the received RF signal into an IF signal based on a reference frequency provided from an LO 508.
A SAW filter 504 passes a predetermined band signal of the down-converted IF signal. An RF up-converter 505 up-converts the passed IF signal into an RF signal based on a reference frequency provided from the LO 508. A high-power amplifier 506 amplifies the up-converted RF signal, and the amplified RF signal is transmitted at the same frequency via a Tx antenna 507.
However, the on-channel repeaters described above with reference to FIGS. 3 to 5 have limited transmission output because of a feedback signal caused by low isolation of the Rx/Tx antennas.
FIG. 6 is a block diagram for explaining an example of a conventional demodulation-type on-channel repeater.
Configurations and operations of the conventional demodulation-type on-channel repeater will now be described with reference to FIG. 6. An RF receiver 602 down-converts an RF signal received via an Rx antenna 601 from a main transmitter or another repeater into a signal of a predetermined. A subtractor 603 removes a feedback signal by subtracting a replica of the feedback signal from the down-converted signal of the predetermined band.
A replica creator 604 creates a replica of the feedback signal based on an output signal of the subtractor 603, i.e., a signal without a feedback signal, and feeds back the created replica to the subtractor 603. An RF transmitter 605 converts the output signal of the subtractor 603, i.e., the signal without a feedback signal into an RF signal, and transmits the RF signal via a Tx antenna 606 by radio.
FIG. 7 is a block diagram illustrating a detailed configuration of the demodulation-type on-channel repeater of FIG. 6.
In FIG. 7, an Rx antenna 701, an RF receiver 702, a subtractor 703, an RF transmitter 706 and a Tx antenna 707 correspond to the Rx antenna 601, the RF receiver 602, the subtractor 603, the RF transmitter 605 and the Tx antenna 606 illustrated in FIG. 6, respectively. Thus, description thereof is omitted.
The replica creator 708 includes a filter coefficient creator 705, and an adaptive filter 704. The filter coefficient creator 705 creates a filter tap coefficient being used at the adaptive filter 704, based on an output signal (i.e., a signal without a feedback signal) of the subtractor 703. The adaptive filter 704 creates a replica of the feedback signal by using the output signal of the subtractor 703 and the filter tap coefficient received from the filter coefficient creator 705, and feeds back the replica to the subtractor 703.
The filter coefficient creator 705 calculates a filter tap coefficient ( hnow) according to the Least Mean Square (LMS) algorithm based on the following Equation 1.
Math Figure 1 hnow= hpast+λ·ē  [Math.1] hnow=[hnow,0 hnow,1 . . . hnow,M−1]T  hpast=[hpast,0 hpast,1 . . . hpast,M−1]T ē=[e0 e1 . . . eM−1]T  [Math.1]
where    ē
denotes an error signal of a channel calculated based on channel distortion information of an estimated repeater reception channel,     hpast 
denotes a previous filter tap coefficient,    λ
denotes a constant that determines a convergence speed, M denotes a filter tap number, and T denotes a transpose.
The adaptive filter 704 filters an output signal vector ( yn=[y(n)y(n−1) . . . y(n−M+1)]T) at a time index (n) outputted from the subtractor 703 on the basis of the filter tap coefficient ( hnow) created by the filter coefficient creator 705, thereby calculating a replica (fb(n)) of a feedback signal based on the following Equation 2.
Math Figure 2fb(n)= hnowT· yn  [Math.2]
The subtractor 703 removes the feedback signal caused by low isolation of the Tx/Rx antennas by subtracting the replica (fb(n)) of the feedback signal calculated at the adaptive filter 704 from the output signal (r(n)) of the RF receiver 702, based on the following Equation 3.
Math Figure 3y(n)=r(n)−fb(n)  [Math.3]
FIG. 8 is a block diagram illustrating a detailed configuration of the filter coefficient creator 705 of FIG. 7.
Referring to FIG. 8, the filter coefficient creator 705 includes a demodulator 801, a channel estimator 802 and a time-domain filter coefficient creator 803.
The demodulator 801 receives an output signal (i.e., a signal without a feedback signal) of the subtractor 703, and demodulates the received signal through frequency and timing synchronization.
The channel estimator 802 estimates channel distortion of a repeater reception channel based on the signal demodulated by the demodulator 801. The channel distortion includes, e.g., a noise, a multi-path signal, and a remaining feedback signal caused by a channel between the main transmitter and the on-channel repeater.
The time-domain filter coefficient creator 803 creates an error signal (ē) in a time domain based on the channel distortion information estimated by the channel estimator 803 to create a filter tap coefficient based on the above Equation 1.
However, the demodulation-type on-channel repeater illustrated in FIG. 6 has performance that largely varies according to a structure of a known pilot signal being used for the feedback channel estimation. Particularly, an interval between pilots is closely related to a time interval of filter coefficient update. Thus, if a system has a long interval between pilots, a feedback signal cannot be effectively removed in the situation where a feedback channel changes rapidly.
Since the conventional repeaters described above have limited capability of removing feedback signals, conventional on-channel repeating systems have limitations of low utilization of existing repeating facilities, and high investment costs.
For that reason, the demodulation-type on-channel repeater of FIG. 6 requires a method of reducing a filter-coefficient update interval for high utilization and low investment costs, which can achieve an identical output signal of the on-channel repeater to an output signal of a main transmitter, a short time delay between the two output signals, a quick response to a changing feedback channel, and an increase in transmission output power of the on-channel repeater by removing a feedback signal caused by low isolation of Tx/Rx antennas of the on-channel repeater.